feat: add MCP enums, ToolError, and Caller

client/error.go

@@ -0,0 +1,28 @@
+package client
+
+import "fmt"
+
+// ToolError is a failed MCP tool call or a parse of its result.
+type ToolError struct {
+	Name    string // MCP tool name
+	Message string
+	Err     error // transport/SDK/parse cause; may be nil
+}
+
+func (e *ToolError) Error() string {
+	return "mcp " + e.Name + ": " + e.Message
+}
+
+func (e *ToolError) Unwrap() error {
+	return e.Err
+}
+
+// ToolErrorf builds a ToolError. format may use %w so Unwrap works.
+func ToolErrorf(name, format string, args ...any) *ToolError {
+	err := fmt.Errorf(format, args...)
+	return &ToolError{
+		Name:    name,
+		Message: err.Error(),
+		Err:     err,
+	}
+}